A technology for inspecting a measurement target includes an optical probing technology for irradiating a measurement target with light emitted from a light source and detecting measurement light (reflected light) from the measurement target using a light sensor to acquire a detection signal. In such an optical probing technology, a method of arranging a magneto-optical (MO) crystal to face a light irradiation surface of the measurement target and detecting reflected light according to a magneto-optical effect of the MO crystal to acquire the detection signal is known (for example, Patent Literature 1).